The invention relates to seafloor electromagnetic surveying for resistive and/or conductive bodies, for example for oil, gas, methane hydrates etc. and other hydrocarbon reserves or subterranean salt bodies.
Seismic techniques are frequently used during hydrocarbon-exploration expeditions, to identify the existence, location and extent of reservoirs in subterranean rock strata. However, whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them. This is especially so for pore fluids which have similar mechanical properties, such as oil and seawater. It is therefore generally necessary to employ other survey techniques to determine whether a previously identified reservoir contains oil, for example, or just aqueous pore fluids. One technique is exploratory well drilling in the region of potential interest. However, this is expensive and time consuming. Alternative techniques include electromagnetic (EM) surveying techniques.
EM surveying techniques seek to distinguish oil- and water-filled reservoirs on the basis of their differing electrical properties. One group of EM surveying techniques are the controlled-source EM (CSEM) survey techniques.
CSEM techniques involve transmitting an EM signal into the seafloor, generally using a horizontal electric dipole (HED) source (transmitter), and measuring the response at EM receivers (detectors) for a range of distances from the source (i.e. for a range of source-receiver offsets/separations). Since hydrocarbons are more resistive than seawater (e.g. hydrocarbon-bearing sediments typically have resistivities on the order of a few tens of Ωm or higher versus a few Ωm or so for water bearing sediments), the presence of a hydrocarbon-bearing reservoir will, in general, lead to stronger EM fields than would be the case if the reservoir contained only seawater. This is because the highly conducting seawater attenuates the component of the EM signal passing through the reservoir more than would be the case if the reservoir contained hydrocarbon. Conversely, the presence of relatively more conductive structures in the subterranean strata will, in general, lead to weaker EM fields seen at the detector. This is because of the increased attenuation of fields in the conductive structure. Thus an analysis of the electromagnetic fields measured during a CSEM survey, e.g. field amplitudes and phases, can in principle provide information on subterranean resistivity profiles, and hence likely reservoir content.
In practice the interpretation of CSEM survey results is not so simple. This is because CSEM surveys are primarily sensitive to the transverse resistance of the subterranean strata (i.e. primarily dependent on the resistivity-thickness product of subterranean layers). Because of this the effects on detected EM fields caused by the presence of a thin resistive layer (e.g. a hydrocarbon reservoir) can be indistinguishable from the effects arising from other realistic large scale subterranean strata configurations. For example, CSEM data from subterranean strata comprising a thin resistive hydrocarbon reservoir embedded in a largely uniform resistivity background can be similar to data from a subterranean strata configuration comprising larger-scale layers having increasing resistivity with depth [1]. This kind of increasing-resistivity structure is a feature of some submarine sedimentary basins, for example, and can arise due to the progressive expulsion of conductive pore fluids with increasing depths by a rising overburden pressure. Because of this possible ambiguity, it is known that data which are primarily sensitive to thin resistive/conductive layers and data which are primarily sensitive to the large-scale background structure of the subterranean strata are needed for a comprehensive CSEM survey [1].
Various different techniques have been proposed to provide suitable differently sensitive data. Fundamentally the techniques are based on the simultaneous interpretation of two contrasting EM datasets. One data set comprises data which are primarily dependent on a transverse electric (TE) mode of coupling between the source and the receiver. The other data set comprises data which are primarily dependent on a transverse magnetic (TM) mode of coupling between the source and the receiver.
EM signals used in CSEM generally comprise TE and TM mode components. The response of seawater and subterranean strata (which will typically approximate a series of planar horizontal layers) to EM signals is generally different for TE mode components of the transmitted signal, which excite predominantly horizontal current flows, and TM mode components, which excite significant components of vertical current flow. For TE mode components, the coupling between horizontal layers comprising the subterranean strata is largely inductive. This means the presence of thin resistive layers (which are indicative of hydrocarbon reservoirs) does not significantly affect the EM fields detected at the surface as the large scale current flow pattern is not affected by the thin layer (since the subterranean fields couple relatively well across the thin layer). However, for TM mode components the coupling between layers includes a significant galvanic component (i.e. due to the direct transfer of charge between layers). Thus for the TM mode even a thin resistive layer strongly affects the EM fields detected at the receiver since the large scale current flow pattern is disrupted by the resistive layer.
Thus to resolve the above-mentioned possible ambiguities in interpreting CSEM data it is known to determine the response of the subterranean strata to both TE mode components (i.e. inductively coupled and most sensitive to large-scale background structure) and TM mode components (i.e. galvanically coupled and more sensitive to thin resistive/conductive layers) [1]. Approaches based on these principles thus provides two complementary data sets having different relative sensitivities to the TE and TM mode components of the transmitted EM signals. Analysis of these complementary data sets can then reveal differences between the TE mode and TM mode coupling between the source and receiver, and these differences are indicative of the presence or not of a subterranean hydrocarbon reservoir.
One proposed way of obtaining respective TE and TM dominated data sets is to obtain data for different relative orientations between a towed HED source and receivers in an array [1]. According to this scheme, data from receivers which are arranged inline with the HED source (i.e. on a line parallel to and passing through the dipole axis of the HED transmitter) are TM mode dominated (sensitive to the presence of thin resistive layers indicative of hydrocarbon-bearing reservoirs), whereas data from receivers which are arranged broadside to the HED source (i.e., on a line perpendicular to and passing through the HED axis), are TE mode dominated (more sensitive to characteristics of the large scale background). This splitting arises from the inherent geometry of the dipole field from the source. However, a drawback of this geometric-splitting based approach is the need to obtain data for multiple specific source-receiver alignments results in complex tow paths. These are costly and time consuming to perform and provide relatively little data of the highest possible quality (since much of the data is neither inline nor broadside but is intermediate between the two).
Another proposed way of obtaining respective TE and TM dominated data sets is to mathematically decompose CSEM data into TE and TM mode components [2, 3]. This can be done for data obtained over a range of source-receiver orientations and so more efficient tow paths and so can employed than for schemes based on geometric-splitting. However, these mathematical decomposition schemes are relatively sensitive to noise in the CSEM data sets, and also require specialised receiver designs which capable of measuring spatial gradients in electromagnetic field components.
Yet another proposed scheme for obtaining respective TE and TM dominated data sets is to collect conventional CSEM inline data (primarily TM mode coupled), and to separately collect data using the passive magneto-telluric (MT) method [4]. In an MT survey, signals at a surface-based electromagnetic detector arising in response to EM fields generated naturally, such as within the earth's upper atmosphere, are measured [5, 6]. The measured responses can provide information about the subterranean rock strata beneath the detectors. Generally all but those MT signals with periods corresponding to several cycles per hour are screened from the seafloor by the overlying highly conductive seawater. The long wavelength signals which do penetrate to the seafloor do not provide sufficient spatial resolution to examine the electrical properties of small scale subterranean reservoirs, but can be used for larger-scale undersea probing. Furthermore the MT signals at the seafloor comprise primarily horizontally polarised EM fields and so are intrinsically insensitive to thin resistive layers. Thus MT data can be used in place of TE dominated controlled source data to provide information on the large scale background subterranean strata configuration.
The CSEM plus MT technique allows the CSEM data to be collected more efficiently than schemes based on geometric-splitting (since only inline data are needed). Furthermore the CSEM data may be collected using conventional receivers. However, a drawback of the CSEM plus MT technique is the need to separately acquire two independent data sets. This is because the CSEM source is switched off to acquire MT data. Furthermore, MT signals are uncontrolled and often weak. This means a significant amount of data stacking can be required to reach an appropriate signal to noise ratio. In some cases several days of MT data must be recorded to ensure signal quality, increasing the time for which receivers must be deployed compared to a standard CSEM survey and therefore the overall commercial cost. Furthermore still, only low frequency data can generally be used as higher frequency signals undergo too much attenuation in seawater overlaying the area being surveyed. This means in practice that good quality data are not acquired in the same frequency band as for the CSEM data. This makes each data set differently sensitive to different regions in the subterranean strata and this increases the complexity of data interpretation.
There is therefore a need for methods of analysing CSEM data which do not suffer from the above-described drawbacks of known techniques.